[chore] update dependencies (#4386)

- codeberg.org/gruf/go-bytesize v1.0.3 -> v1.0.4
- codeberg.org/gruf/go-kv/v2 v2.0.6 -> v2.0.7
- codeberg.org/gruf/go-mutexes v1.5.2 -> v1.5.3
- codeberg.org/gruf/go-structr v0.9.7 -> v0.9.8
- codeberg.org/gruf/go-ffmpreg v0.6.8 -> v0.6.9
- github.com/tomnomnom/linkheader HEAD@2018 -> HEAD@2025

all of the above codeberg.org/gruf updates are in preparation for Go1.25, except for bytesize, and also ffmpreg which is a rebuild with the latest version of ffmpeg (v5.1.7)

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4386
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

vendor/codeberg.org/gruf/go-mangler/v2/helpers.go

@@ -0,0 +1,43 @@
+package mangler
+
+import (
+	"unsafe"
+)
+
+func append_uint16(b []byte, u uint16) []byte {
+	return append(b, // LE
+		byte(u),
+		byte(u>>8),
+	)
+}
+
+func append_uint32(b []byte, u uint32) []byte {
+	return append(b, // LE
+		byte(u),
+		byte(u>>8),
+		byte(u>>16),
+		byte(u>>24),
+	)
+}
+
+func append_uint64(b []byte, u uint64) []byte {
+	return append(b, // LE
+		byte(u),
+		byte(u>>8),
+		byte(u>>16),
+		byte(u>>24),
+		byte(u>>32),
+		byte(u>>40),
+		byte(u>>48),
+		byte(u>>56),
+	)
+}
+
+func empty_mangler(buf []byte, _ unsafe.Pointer) []byte {
+	return buf
+}
+
+// add returns the ptr addition of starting ptr and a delta.
+func add(ptr unsafe.Pointer, delta uintptr) unsafe.Pointer {
+	return unsafe.Pointer(uintptr(ptr) + delta)
+}